writeObject

Save the state of the ConcurrentHashMap instance to a stream
(i.e., serialize it).

Serial Data:

The capacity of the ConcurrentHashMap(the length of
the bucket array) is emitted(int), followed by the size of the
ConcurrentHashMap(the number of key-value mappings), followed by the key
(Object) and value(Object) for each key-value mapping represented by the
ConcurrentHashMap The key-value mappings are emitted in the order that
they are returned by entrySet().iterator().